An inherent characteristic of an air conditioning compressor, such as used in an automotive air conditioning system, is the generation of vibration due to the dynamics of the compression process, the interaction of gaseous refrigerant flow between the cylinders and the compressors, and the physical characteristics of the moving parts. These vibrations have the undesirable effect of creating objectional noise and/or destructive forces when the compressor RPM causes vibration at the resonant frequency of the system, thereby causing the dangerous occurrence of resonance. Also, the vibrating components are prone to more rapid wear and premature failure.
It has been found that the specific design and construction of the torque cushion of the air conditioning compressor's electromagnetic clutch assembly contributes significantly to the frequency at which the compressor will resonate. The torque cushion is a resilient member disposed between the clutch armature and the drive shaft of the compressor. When the electromagnetic clutch is energized, a magnetic flux flowing through the armature plate pulls the armature plate against a rotating pulley to initiate rotation of the drive shaft. The moment at which the armature plate is engaged causes a significant instantaneous increase in the torsional stress of the drive shaft. Extreme and repeated increases in the torsional stress of the drive shaft contribute to premature failure.
To alleviate this problem, the prior art teaches the use of elastomeric, e.g., rubber, torque cushions which absorb torsional stresses upon engagement of the armature plate to the pulley and thereby protect the drive shaft from frequent loading at extremely high torsional stresses. Examples of the prior art elastomeric torque cushions are shown in the U.S. Pat. No. 3,384,213 to Bernard et al, issued May 21, 1968 and assigned to the assignee of the subject invention, and U.S. Pat. No. 4,624,354 to Koitabashi, issued Nov. 25, 1986.
Both of these prior art references disclose an annular elastomeric torque cushion having an axial thickness which is substantially constant throughout its entire radial extent. These elastomeric torque cushions have several deficiencies. Firstly, the shear stress exhibited in the elastomeric material of the torque cushion is significantly larger near the drive shaft. Hence, upon each engagement of the armature plate to the pulley, extremely high shear forces are placed upon the elastomeric material near the center of the torque cushion. Accordingly, failure of the torque cushion due to torsional stress will occur in those areas of highest shear stress. Secondly, the constant thickness elastomeric torque cushions do not have the axial resiliency needed when the armature plate is drawn against the pulley. During these times, the torque cushion acts as an axial spring by flexing in the axial direction to permit movement of the armature plate into engagement with the pulley. The stiffer, i.e., more resistant, the axial deflection of the torque cushion, the less friction is established between the armature plate and the pulley and hence the more likely slippage will occur. Thirdly, the prior art elastomeric torque cushions are known to significantly contribute to the establishment of a resonant frequency of the compressor. Of the prior art elastomeric torque cushions, resonance will occur within the normal operating speed, e.g., within the 3000 to 4000 RPM range, of the compressor, which, by way of reference, is usually about one and one half time greater than the RPM of the automotive engine. Hence, during normal operation, the prior art compressors will vibrate and shake at their resonant frequency causing undesirable noise and fatigue on the components thereof.